Most raw materials such as crude oil are typically mixtures composed of a number of compounds. Such raw materials are rarely used without purification in industrial fields, and in most cases, they are separated into individual compounds before use. Distillation is a representative chemical process for separating mixtures into their respective components.
Generally, distillation serves to separate higher boiling components and lower boiling components from each other. (n−1) Distillation columns are required for the separation of a feed mixture composed of n components, where the number of columns is one larger than that of the components of the mixture. That is, in many cases, a conventional distillation process for the separation of a three-component mixture into its individual components employs a continuous two-column distillation system.
FIG. 1 illustrates a conventional two-column distillation system for separating a three-component mixture into the individual components.
Referring to FIG. 1, the distillation system comprises a first column 11, where a component D having the lowest boiling point is separated from a component S having the intermediate boiling point and a component B having the highest boiling point, and a second column 21, where the components S and B are separated from each other.
The composition profile in the first column is shown in FIG. 2. As shown in FIG. 2, remixing of the intermediate boiling component (S) usually occurs in the lower portion of the first column.
The conventional distillation process is advantageous in controlling the compositions of the products, but remixing of the intermediate boiling component in the first column takes place. This remixing results in low thermodynamic efficiency of the distillation column system, bringing about unnecessary additional energy consumption.
In order to solve such problems, a great deal of research has been conducted on novel distillation systems. As a representative example, a Petlyuk distillation column for improving the separation efficiency of a feed mixture, which consists of a low boiling component, an intermediate boiling component and a high boiling component, by a thermally coupled structure, is illustrated in FIG. 4. The Petlyuk distillation column comprises a preliminary separator 12 and a main separator 22, which are arranged in a thermally coupled structure. The low boiling component and the high boiling component are primarily separated from each other in the preliminary separator, and then the top and bottom products are introduced into respective feed plates of the main separator, where the low boiling component, the intermediate boiling component and the high boiling component are separated from each other. This structure increases the energy efficiency of the Petlyuk distillation column because the distillation curves in the Petlyuk distillation column become similar to the equilibrium distillation curve. However, the column is not easy to design and operate. Particularly, it is difficult to balance the internal pressures of the columns.
To overcome the limitations of Petlyuk distillation columns, dividing wall distillation columns (DWCs) have been proposed. A dividing wall distillation column is similar to a Petlyuk distillation column from a thermodynamic viewpoint, but they are structurally different from each other. A typical dividing wall distillation column has a structure in which a dividing wall is installed to integrate a preliminary separator and a main separator of a Petlyuk distillation column with each other. This structure solves the difficulties of the Petlyuk distillation column, i.e. a difficulty in balancing the pressures of the preliminary separator and the main separator of the Petlyuk distillation column and a difficulty in operating the Petlyuk distillation column. In addition, the integration of the two separators greatly lowers the investment cost of the Petlyuk distillation column.
Some distillation techniques for the production of n-butanol can be found in Korean Patent Publication No. 10-2003-0088211 A2 published on Nov. 19, 2003 (‘Patent Publication 1’) and Korean Patent Publication No. 10-2008-0099034 A1 published on Nov. 12, 2008 (‘Patent Publication 2’).
Patent Publication 1 suggests a method for purifying n-butanol using only two distillation columns. Specifically, the method comprises adding an alkaline additive to slop butanol as a raw material, which is a side product created during the production of oxoalcohol, removing water and low boiling materials from the mixture in a first distillation column 1, and removing high boiling materials from the remaining mixture in a second distillation column 2. According to the method, since the two-column distillation system produces n-butanol in an amount comparable to that of n-butanol produced by the operation of a conventional three-column distillation system, the number of processing steps is reduced and the energy and cost required to operate the two-column distillation system are greatly reduced in comparison to those of the conventional three-column distillation system.
Patent Publication 2 proposes a method comprising introducing a mixture consisting of a low boiling material A, an intermediate boiling material B and a high boiling material C into a first distillation column; separating the mixture in the first distillation column to provide a top product and a bottom product of the first distillation column to prevent remixing of the intermediate boiling material in the lower portion of the first distillation column; and separating the bottom product in the second distillation column to provide a top product and a bottom product of the second distillation column, thereby controlling the concentration ratio between the intermediate boiling material B and the low boiling material A in the top product of the second distillation column.